1. Field of the Invention
This invention relates to a cooling apparatus for use in internal combustion engines.
2. Description of Related Art
Various proposals have been made regarding control of coolant temperature in internal combustion engines. For example, Japanese Patent Application Publication No. 60-81422 (JP 60-81422 A) discloses a control mechanism for an automobile electric-powered fan, which is designed to increase the rotational speed of the electric-powered fan with increase in coolant temperature.
In the meantime, there has recently been proposed to control the coolant temperature for the purpose of improving the fuel efficiency by suppressing knocking. Specifically, in order to suppress knocking, the coolant temperature is controlled to be lower than a common coolant temperature to suppress knocking so that the fuel efficiency is improved. When the coolant temperature is normally set lower in this manner, the ignition timing also need be adjusted accordingly.
When such coolant temperature control is used as a measure for improving the fuel efficiency, it may possibly induce problems as follows. When a vehicle enters an idling state or low-speed traveling state, the effect of cooling a radiator by running wind is reduced. This may result in increased coolant temperature. If the vehicle is rapidly accelerated after the increase of the coolant temperature, knocking may likely occur due to the ignition timing that is adjusted according to the coolant temperature set lower than normal. When an internal combustion engine is provided with a knock control system (KCS), a learning function of the KCS is activated once occurrence of knocking is detected. When the retardation amount at a low coolant temperature is increased by the learning function, it may possibly induce deterioration of fuel efficiency performance, or deficiency in motivity performance such as slow acceleration.
The control mechanism for an automobile electric-powered fan disclosed in JP 60-81422 A is designed to control rotation of the electric-powered fan according to increase in coolant temperature. According to this technique, in which control is performed with reference to the coolant temperature, the coolant temperature might be increased at some timing due to delayed response of the coolant temperature control, making it impossible to prevent occurrence of knocking. In addition, if the fan is activated frequently in order to keep the coolant in a low temperature state as much as possible, the fuel efficiency might be rather deteriorated due to increased power consumption by the fan or increased driving resistance. Furthermore, the frequent activation of the fan may possibly cause a problem of activation noise or increase of variation range of the coolant temperature. Still further, frequent activation of the fan will reduce the lifetime of the fan itself.